6.7 The override Directive

If a variable has been set with a command argument
(see Overriding Variables),
then ordinary assignments in the makefile are ignored. If you want to set
the variable in the makefile even though it was set with a command
argument, you can use an override directive, which is a line that
looks like this:

Variable assignments marked with the override flag have a
higher priority than all other assignments, except another
override. Subsequent assignments or appends to this variable
which are not marked override will be ignored.

The override directive was not invented for escalation in the war
between makefiles and command arguments. It was invented so you can alter
and add to values that the user specifies with command arguments.

For example, suppose you always want the ‘-g’ switch when you run the
C compiler, but you would like to allow the user to specify the other
switches with a command argument just as usual. You could use this
override directive:

override CFLAGS += -g

You can also use override directives with define directives.
This is done as you might expect: